This invention relates to the electrodeposition of electrically insulating coatings on electrically conducting parts, and in particular, it relates to an apparatus for the inspection of the quality of such coatings by means of electrical measurements.
One type of presently used apparatus for inspecting electrodeposited electrically insulating coatings is shown in FIG. 1. In the figure, reference numeral 1 is an electrically conducting part which has been coated by electrodeposition with an electrically insulating coating. Reference numeral 2 designates is a conveyor for transporting the part 1 during the inspection process. The part 1 is suspended from the conveyor 2 by a suspending device 3 connected to the conveyor 2. Reference numeral 4 designates a tank filled with an electrically conducting liquid 5. Reference numeral 6 designates an elevating device for raising and lowering the tank 4 containing the electrically conducting liquid 5. The elevating device 6 comprises a column 6a mounted on a base 6b, a chain 6c connected to the tank 4, and a motor (not shown) for driving the chain 6c, thereby raising or lowering the tank 4.
The operation of this apparatus is as follows. The conducting part 1 covered with an insulating coating is positioned directly above the tank 4 by the conveyor 2. The tank 4 is then raised automatically by the elevating device 6 until the part 1 is almost completely immersed in the conducting liquid 5. A voltage is then applied between the conducting part 1 and the side of the tank 4 for a predetermined length of time, and the resulting current is measured. The level of the current and the rise time of the current indicate the quality of the insulating coating; if the coating completely covers the conducting part 1, no current will be able to flow between the part 1 and the side of the tank 4. On the other hand, if the coating is very irregular or contains pin holes, the conducting liquid 5 will quickly penetrate to the conducting part 1, and a large current will result. After a predetermined length of time, the tank 4 is lowered and the part 1 is carried off by the conveyor 2 in the direction shown by the arrow.
FIG. 2 shows another commonly used inspection apparatus. It is similar in construction to the apparatus of FIG. 1 and operates on basically the same principles. It differs only in that the tank 4 does not move up and down and in that it is not necessary to stop the forward motion of the part 1 in order to inspect the insulating coating. The conveyor 2 is shaped so that the part 1 is dipped into the conducting liquid 5 during its forward motion. While it is passing through the liquid 5, a voltage is applied between the part 1 and the side of the tank 4, and the current flowing between the two is measured.
Unfortunately, inspection carried out using the apparatuses shown in FIGS. 1 and 2 is often inaccurate. These apparatuses include no means for preventing air bubbles from adhering to the part 1 being inspected. Whereas the conducting liquid 5 should penetrate to the conducting part 1 in locations where the insulating coating is inadequate, if air bubbles adhere to these locations, the liquid 5 will be prevented from penetrating. Accordingly, the current flowing between the part 1 and the side of the tank 4 will be misleadingly low and will not give a correct indication of the quality of the insulating coating.
Furthermore, both apparatuses have the drawback that they entail both horizontal and vertical movement, which makes structure of the apparatuses unnecessarily complicated and increases their cost.
Another method of inspecting insulating coatings is to apply a thin, electrically conducting foil to the part 1 to be inspected, apply a voltage between the part 1 and the foil, and measure the dielectric strength of the insulating coating. However, the application of the foil requires considerable effort, and this method is not suitable for mass production.